Many refining and chemical process reactors employed in the petroleum chemical industry employ the flow of a vapor and liquid mixture in a downward direction through one or more fixed beds of catalyst contained within a reactor vessel. These reactors are generally operated in the trickle flow regime meaning that the flowing gas forms a continuous phase that fills up the space between the catalyst particles while the liquid trickles down the bed of catalyst particles in the form of liquid rivulets and liquid films. In order to evenly distribute the incoming gas-liquid mixture across the catalyst bed, a flow distributor tray is used above each catalyst bed. The distributor tray divides the incoming liquid into a plurality of small streams.
One of the major problems in the design and operation of trickle bed reactors is that the catalyst particles are not fully wetted by the trickling down liquid. Dry spots exist in the catalyst bed where the catalyst is deprived of the liquid reactants. Thus, because a part of the catalyst cannot participate in the reaction, the catalyst is underutilized.
In order to overcome the problem of incomplete catalyst wetting and utilization, the trickle bed reactors are normally designed at high liquid mass velocity (high liquid flow rate per unit cross section area of the reactor). A high liquid mass velocity leads to a large number of liquid rivulets and films flowing down the catalyst bed, thereby giving a more complete catalyst wetting and utilization. A high mass velocity means that the cross section area or diameter of the reactor vessel be small. A small diameter implies that the reactor height be relatively high in order to get within the reactor the required volume of catalyst. Thus, it has been conventional in the design of reactor vessels to prefer the small diameter higher height reactor geometry to assure catalyst wetting. Alternatively, the reactor could be designed relatively big in diameter and shorter in overall height with the same volume of catalyst therein and process capacity. Such larger diameter reactor design will have a necessarily lower flow rate per unit reactor cross-section area for the same capacity but with the attendant drawback that some of the catalyst particles in the catalyst bed may remain unwetted and therefore not participate in the reaction.
The high mass velocity reactors (small diameter and higher height reactors) while having the advantage of better catalyst wetting, have the disadvantage of high pressure drop. The problem of high pressure drop becomes particularly severe if the reactor contains catalyst particles which are of small size. The problem of high pressure drop is that it is energy inefficient, but in addition is undesirous when processing petroleum feeds which are heavy or dirty in composition and therefore, more prone to plugging of the reactor beds. Accordingly, it is desirable from an energy conservation standpoint, as well as ability to process heavier and dirtier process feeds, to design reactors which are relatively larger in diameter and shorter in height than previous design practices have permitted.